The invention relates to a colour selection means for colour display tubes, comprising a curved mask which is secured to a frame, which frame comprises a plurality of interconnected parts each forming a side of the frame.
A colour display tube with a colour selection means usually comprises a display screen having a pattern of lines or dots of an electroluminescent material, and an electron gun for generating electron beams, in which the colour selection means, which may comprise a wire mask or an apertured mask, is arranged between the electron gun and the display screen. The mask is a thin metal foil having a large number of apertures and should be arranged very accurately and very close to the display screen so that the mask apertures are systematically aligned with respect to the luminescent lines or dots on the display screen. To maintain the systematic alignment, the mask should be suspended in the colour display tube in an appropriate manner in order that the mask remains in an accurately determined position both during its manufacture and during operation of the tube. To realise this, a frame supporting the mask and being suspended in the tube is used. The frame is generally rectangular, but may alternatively be, for example square-shaped or elliptical.
In the case of a mask having a single (for example, cylindrical) curvature a frequently used construction is one in which a rigid and heavy frame is provided with the correct curvature, whereafter the mask is secured at the curved sides. In the case of a mask having a double curvature an example of a frequently used construction is the one in which the supporting frame comprises four L-shaped metal parts whose ends are interconnected and thus constitute a rectangular frame. See, for example Funkschau 11, 1987, pp. 49-54 ("Neue Masken fur hellere Bilder"). In this case the desired accuracy is achieved by giving the mask the correct shape in advance (by means of deep-drawing) so that it will have a sufficient rigidity of itself. Reaching the desired accuracy by means of a metal-cutting finishing operation of a rigid and heavy frame, as well as deep-drawing of the mask has its drawbacks.